The shelf life of electrolyte compositions for capacitors has been a problem for over 40 years. Most of the research has been directed to the formation of an adequate seal for the oxide film on the anode electrode which is covered by a skin or film of an oxide produced by anodic oxidation which functions as a dielectric. The purpose of the seal is to make the anode less susceptible to degradation by hydration of the oxide.
B. Chesnot, in U.S. Pat. Nos. 3,546,119 and 3,638,077, discloses an electrolyte for single and multiple aluminum electrode capacitors comprising about 0.5 to 10 moles of boric acid per 11 moles of a solvent such as gamma-butyrolactone, gamma-valerolactone and N-methylpyrrolidone. Chesnot's compositions can contain between 0.1 to 2 moles of a trialkylamine per mole of the solvent. The anion generating agent may be such acids as acetic, acrylic, butyric and citric acids. These compositions are substantially non-aqueous but are dependent on the presence of a small amount of water necessary to reform the anode (oxide) layer produced in situ by the dehydration of boric acid into metaboric acid and water. Without this water balance the electrolyte is subject to premature destruction through self-sustained elevation of the leakage current or an unwanted change in the electrical characteristics of the capacitor. Such compositions also require the presence of the afore-mentioned anode-generating acids in order to attain lowered resistivity. Ethylene glycol is incorporated in the compositions of the '077 patent as a solubilizer.
U.S. Pat. No. 3,835,055, by B. Chesnot, discloses an aqueous electrolyte for aluminum-anode capacitors which is composed of maleic acid, a maleate of N-methylbutylamine, triethylamine or tributylamine and phosphoric acid dissolved in dimethylformamide (DMF) wherein the maleic acid and the maleate salt are present in an amount between 10.sup.-3 moles up to their up limits of solubility per mole of solvent (DMF) and the phosphoric acid is present in an amount between 10.sup.-3 to 10.sup.-1 moles per mole of solvent (DMF).
These compositions are described as being very slightly aqueous and function where low temperatures, high frequencies and possibly high temperatures in the order of at least 85.degree. C. are encountered. Stability over a temperature range of -55.degree. C. to +125.degree. C. is disclosed. High faradic efficiency of the anodic oxidation and a limited service voltage, i.e. with maximum build-up voltage, are disclosed by Chesnot. Tests show that the combination of the maleic acid and its salt produces an electrical conductivity (mmho/cm at 35.degree. C.) which is more than additive and that the addition of phosphoric acid raised the build-up voltage without affecting the electrical conductivity by more than 10%. Also, a small amount, i.e. 0.01% to 3% by weight of water and various lactones and DMF-ethylene glycol solvent combinations can be used with the phosphoric acid being an essential ingredient.